Top-down Program Design, Relational and Logical Operators Selim Aksoy Bilkent University Department of Computer Engineering saksoy@cs.bilkent.edu.tr

Creating MATLAB Scripts Choose File>New>M-file from the menu Use the editor to write your program Document your program using comments that include Short note about what your program does Short note about how it works Author information Date information Version information Fall 2004 CS 111

Creating MATLAB Scripts % Script file: temp_conversion.m % % Purpose: % To convert an input temperature from degrees Fahrenheit to % an output temperature in kelvins. % Record of revisions: % Date Programmer Description of change % ==== ========== ===================== % 12/01/97 S. J. Chapman Original code % Define variables: % temp_f -- Temperature in degrees Fahrenheit % temp_k -- Temperature in kelvins % Prompt the user for the input temperature. temp_f = input('Enter the temperature in degrees Fahrenheit: '); % Convert to kelvins. temp_k = (5/9) * (temp_f - 32) + 273.15; % Write out the result. fprintf('%6.2f degrees Fahrenheit = %6.2f kelvins.\n', ... temp_f,temp_k); Fall 2004 CS 111

Top-down Program Design State the problem Start Define inputs and outputs Decomposition Design the algorithm Stepwise refinement Convert algorithm into MATLAB statements Test the resulting program End Fall 2004 CS 111

Pseudocode A hybrid mixture of MATLAB and English for defining algorithms Independent of any programming language so it can be easily converted to any programming language Example pseudocode: Prompt user to enter temperature in degrees Fahrenheit Read temperature in degrees Fahrenheit (temp_f) temp_k (in Kelvins)  (5/9) * (temp_f – 32) + 273.15 Write temperature in degree Kelvins Fall 2004 CS 111

Top-down Program Design Problem: write a program that takes the radius and height (in meters) of a cylinder tank and the amount of water (in m3) from the user and output the amount of extra space (in m3) in the tank. Input: radius and height amount of water Output: extra space Fall 2004 CS 111

Top-down Program Design Get radius of the tank base from the user Get the height of the tank from the user Get the amount of water Calculate the amount of extra space Write the result Step 4 is not clear enough, refine it: Calculate the capacity of the tank (pi * radius^2 * h) extra space  capacity - water Fall 2004 CS 111

Top-down Program Design Code: r = input('Enter the radius of the tank base:'); h = input('Enter the height of the tank:'); water = input('Enter the amount of water:'); capacity = pi * r^2 * h; space = capacity - water; fprintf('There is %f m3 extra space in the tank', space); Fall 2004 CS 111

Top-down Program Design Testing: Enter the radius of the tank base:2 Enter the height of the tank:5 Enter the amount of water:10 There is 52.831853 m3 extra space in the tank Continue testing: Enter the amount of water:100 There is -37.168147 m3 extra space in the tank Fall 2004 CS 111

Top-down Program Design Design: refine step 4 again Calculate the capacity of the tank (pi * radius^2 * h) extra space  ((capacity – water) + abs(capacity – water))/2 Fall 2004 CS 111

Relational Operators Relational operators are used to represent conditions (such as “space  0” in the water tank example) Result of the condition is either true or false In MATLAB: false is represented by 0 true is represented by 1 (non-zero) Fall 2004 CS 111

Relational Operators Operation Result 3 < 4 1 3 <= 4 3 == 4 3 ~= 4 3 > 4 4 >= 4 ‘A’ < ‘B’ Fall 2004 CS 111

Relational Operators Don’t confuse equivalance (==) with assignment (=) Be careful about roundoff errors during numeric comparisons (you can represent “x == y” as “abs(x-y) < eps”) Relational operations have lower priority than arithmetic operations (use parentheses to be safe, though) Fall 2004 CS 111

Logical Operators More complex conditions can be represented by combining relational operations using logic operators Logical operators: & AND | OR xor Exclusive OR ~ NOT Fall 2004 CS 111

Logical Operators input and or xor not a b a & b a | b xor(a,b) ~a 1 1 Fall 2004 CS 111

Operator Hierarchy Processing order of operations: parenthesis (starting from the innermost) exponentials (left to right) multiplications and divisions (left to right) additions and subtractions (left to right) relational operators (left to right) ~ operators & operators (left to right) | operators (left to right) Fall 2004 CS 111